The Precision Approach Path Indicator (PAPI) provides the pilot with a safe and accurate glide slope on landing approach to the runway. A typical PAPI system consists of four Light Housing Assemblies (LHA's), placed in a row perpendicular to the approach path. Each LHA sends out a beam of light which is usually white above the central horizontal axis and red below the axis. Two LHAs are aimed at either slightly higher than the landing approach angle and two LHAs are aimed at slightly lower. From the pilot perspective, the PAPI appears as combinations of red and white to indicate a path that is too high, too low or correctly on slope. The US Federal Aviation Administration (FAA) has issued strict standard for PAPI systems in the form AC 150/5345-28F and FAA_E—3007 document.
Incandescent, fluorescent or arc lamp based light sources have been employed in the previous disclosed PAPI system. Some examples can be found in U.S. Pat. No. 4,064,424 to Hergenrother, U.S. Pat. No. 4,183,078 to Kidd, U.S. Pat. No. 4,430,695 to Payne et al., and U.S. Pat. No. 4,532,512 to Tanner.
The lamp based PAPI system including those listed above suffers from a short life span of less than 2,000 hours. To solve this problem, laser based PAPI system has been introduced in U.S. Pat. No. 5,287,104 to Shemwell. In the system according to Shemwell, three differently-colored beams of laser light are produced and transmitted, one beam of laser light in a geometric plane containing the aircraft approach direction, another beam to one side of the geometric plane, and the third beam on the other side. The pilot can determine whether the aircraft is on the geometric plane or to which side by the color of the light received. If desired, at least one of the colored laser beams that is transmitted toward one side of the geometric plane can be broken into a plurality of adjacent fan-shaped beams, the light in at least one of the fan-shaped beams being interrupted intermittently. However, the laser based VASI or PAPI apparatus typically suffer from high cost and low luminous power, or in other words, low brightness.
The US Patent Application No US2008/0007430 describes a visual navigational aid apparatus based on high intensity red and white LEDs. The two different colored light beams are projected to space separated through Fresnel lens and slotted aperture, thus providing approach guidance. While the high intensity LED apparatus solves the shortcomings associated with the laser based PAPI apparatus, this design suffers from the low imaging quality of Fresnel lens, low optical efficiency, and large overall system size.
Therefore a need exists for an improved PAPI system which offers a combination of long life time, low cost, high luminous power, compact size, and high reliability.